The present invention relates to a gyrator for converting an inductive load into an apparent capacitive load.
The gyrator is discussed in the following articles: (1) The Gyrator, A New Electric Network Element by B. D. H. Tellegen, Phillips Research Reports, Vol. 3, pp. 81-101, 1948, and in (2) An Electronic Gyrator, by H. O. Voorman et al, IEEE Journal of Solid-State Circuits, Vol. SC-7, pp. 469-474, Dec. 1972.
The gyrator discussed in the second article is illustrated in FIG. 1 in the form of an equivalent circuit. As shown, a couple of voltage-current converter circuits are contained in the gyrator. A couple of PNP transistors Q1 and Q2 and a resistor R1 make up a first voltage-current converter. A couple of NPN transistors Q3 and Q4 and a resistor R2 make up a second voltage-current converter. The bases of the PNP transistors Q1 and Q2 are coupled with input and output terminals 11 and 12, respectively. The resistor R1 connects the emitters of the transistors Q1 and Q2. The collectors Q1 and Q2 are connected to the bases of the NPN transistors Q4 and Q3, respectively. The resistor R2 connects the emitters of the transistors Q3 and Q4. A capacitor C1 is connected between the bases of the transist Q3 and Q4.
In operation, a potential difference v between the bases of the transistors Q1 and Q2 causes a voltage v ' to appear across the resistor R1. If the base-emitter voltage V.sub.BE of those transistors is negligible, v is nearly equal to v'. Current i, caused to flow through the resistor R1 by the voltage v', has the flow direction as indicated by an arrow. With this current i, the capacitor C1 is charged up to voltage Vc. The voltage Vc renders the transistors Q3 and Q4 conductive. The voltage Vc causes current i.sub.c to flow through the resistor R2 in the direction of the arrow. Thus, this circuit operates as a gyrator for converting a capacitve load into an inductive load.
The above gyrator suffers from some restrictions in use and hence in its application. For example, the voltages applied to the input and output terminals must be in phase. The input voltage must be AC. Widening of the input voltage range is accompanied by circuit complexity. One of the possible approaches to widen the input voltage range is to connect a current mirror circuit to the collectors of the transistors Q1 and Q2. The measure still involves problems to be solved. The first is the circuit's complexity. The second is that the operating range of the current mirror circuit is limited within the power source voltage.
This type of gyrator has an attractive application in that the cascade connection of two gyrators provides an equivalent transformer. However, the application is also unsatisfactory since the input and output circuits of the combined gyrators are inseparable from each other.